1. Field of the Invention
The present invention relates to a semiconductor laser module provided with an optical isolator used in optical communications systems.
2. Description of the Prior Art
As optical communication is coming into wider use recently, semiconductor laser modules are required to be lower in price and smaller in size. In an attempt to meet these requirements, for example, Yamauchi et al. described a semiconductor laser module which can be made smaller in size in a presentation titled "Surface-Mounted Optical Circuit Module Suited For Automatic Assembly" in 1995 general conference of The Institute of Electronics, Information and Communication Engineers. The semiconductor laser module described in this presentation does not require the adjustment of optical axis, and therefore allows for cost reduction and mass production.
In an optical communications system, there occurs such a problem that light emitted by a semiconductor laser diode chip is reflected from an input end or an output end face of an optical fiber, which transmits the light, and returns to the semiconductor laser diode chip, thus spreading the spectrum of the laser light and/or increasing noise. In order to reduce the reflected light in the prior art systems, simple measures have been taken such as AR coating and oblique cutting of the optical fiber at the end. As more essential means, it is effective to provide optical isolator between the semiconductor laser diode and the optical fiber.
However, when an optical isolator is used, there has been a problem such that, in addition to the high cost of the optical isolator, lenses are required an front and at the back of the optical isolator resulting an increased number of components which inhibits cost reduction of the system as a whole. There has also been such a problem that the use of the optical isolator requires complicated adjustment of the optical axis which increases the cost in terms of an adjustment operation. Further, there has been a problem such that integrating the optical isolator and the lenses into the optical system inhibits size reduction of the system.